A Researcher Wanted to Determine If Using an Octane Booster $X _ { 1 }$

Multiple Choice

A researcher wanted to determine if using an octane booster would increase gasoline mileage. A random sample of seven cars was selected; the cars were driven for two weeks without the booster and two weeks with the booster. Use the definitions of $X _ { 1 }$ and $X _ { 2 }$ as given in the table. Consequently, $D = X _ { 1 } - X _ { 2 }$ .


Compute the standard deviation of the differences.

A) 0.84
B) 0.13
C) 0.11
D) 0.78